Research On Scour Hole Depth And Energy Dissipation In Cushion Pool Of Free Overfall

The scour damage problem in free overfall are the hotspots and difficulties at home and abroad scholars.Firstly,this paper reviewed the theoretical,experimental and numerical simulation research status of free overfall and scour hole depth.Taking a practical engineering as background and testing with model which was produced by plexiglass.Then,a calculation formula for the scour hole depth from the characteristics of free overfall is deduced.Though the comparative analysis of the model test result and practical engineering with other two existed hole depth calculation formulas,verify the correctness of the formula which is proposed by this paper.Finally,using FLUNTE software to make the numerical simulation of the free overfall of practical engineering.Based on the research obove,the main conclusions are as follows:(1)Comparing the scour hole depth from the energy dissipation viewpoint and the hydraulic jump viewpoint with the measured results of the model test,the results show that the calculated values obtained by the energy dissipation point are larger than the measured values of model test,and the calculated values obtained by the hydraulic jump viewpoint are smaller than the measured values of model test.(2)In this paper,a formula for calculating the scour hole depth was deduced which starts from the characteristics of free overfall.This formula considered the shrinkage of nappe and used the energy conservation of sand grain in the stabilization soure hole to establish equilibrium equations.The calculated values obtained by this formula which is in good agreement with the model test curve.(3)Using the soure hole formula obtained by energy dissipation viewpoint,water jump viewpoint and the formula which deduced by this paper to calculate the soure hole depth of practical free overfall engineering,the results show that the calculated values obtained by the energy dissipation viewpoint formula and the water jump viewpoint formula are both smaller than the measured values of engineering.However,the calculated values obtained by this paper deduced is very close to the measured values of engineering.Though comprehensive comparison of model test and practical engineering,it is illustrated that use the formula of scour hole which was deduced by this paper to calculate the depth of the scour hole in this kind of free overfall is quite rightly.(4)Making the numerical simulation of a practical free overfall engineering and calculating the maximum bottom velocity through the method of presetting water cushion pool with different depths.The results show that the setting of cushion pool can obviously reduce the maximum bottom velocity of the nappe,and the maximum bottom velocity willdecrease with the increase of the cushion pool depth.On this basis,combined with energy dissipation and engineering investment to comprehensive consideration,the layout of cushion pool with 0.9m of this free overfall engineering can satisfy the good energy dissipation effect.Meanwhile,the economy will achieve a comparatively better.This can be used as a repair project to alleviate downstream scour.The method of using the numerical simulation to calculate the maximum bottom velocity to layout the cushion pool can also be applied to other actual projects.Providing reference for engineering design or repair and helping solve the scour problem of such free overfall.